I went thru a lot of the same issues. This is what I came up with.....
http://inov8tr.blogs.com/scooby/2006...b_solutio.html
I have a hub that powers the logic via the USB connection and the bus power powers the devices. I decided the power drain was so minimal for the logic that it was fine to leave on with the sleeping mac mini, as the usb ports are still powered during sleep. I have left my car unstarted for 3-4 days without issue.
My setup has been installed for 9 months now and there has been zero issue with the usb hub and devices connected to it. Let me know if you have any questions....
-INOV8Tr